RCD (residual current device) is designed to prevent dangerous effects on humans and animals of electric current when touching current-carrying and other parts of devices and electrical installations that are energized. The next important function of the device is to prevent fires when leakage currents to the ground appear. The protective action is manifested in the disconnection of the mains supply in the following situations:
- short-circuiting the housing of an electrical appliance under voltage through the body to the ground;
- contact of current-carrying elements with grounded non-current-carrying parts of electrical installations as a result of insulation damage;
- changing ground (PE) and neutral (N) conductors in the electrical circuit.
RCD also protects networks from power surges. To do this, a non-linear resistance is connected to the neutral at the input of the device and the phase at the output. A differential current flows through it when the voltage rises above 270 V, after which the RCD turns off.
Protective devices differ in types and principles of operation. One of the most practical is the selective RCD, which provides targeted disconnection of groups of loads. Its feature isunderestimated speed response characteristic (type S or G). It is installed closer to the source, has a differential trip current rating of 100 or 300 mA, and guarantees that the next conventional RCD upstream of the consumer will trip first.
Thus, modern power grid protection is based on identifying faults and disconnecting individual sections from systems operating in normal modes.
How does an RCD work?
RCD is also called a residual current circuit breaker. The purpose remains the same: to turn off the circuit when a current leakage occurs. The main element of the device is a toroidal transformer with several turns of neutral and phase wires connected in opposite directions. The resulting magnetic field during normal operation of the instrument remains zero. Leakage to the ground upsets the balance, a voltage arises in the secondary winding, upon reaching a certain value, the electrical circuit is switched off using the starting and actuator mechanisms.
An RCD needs a PE ground bus. Otherwise, when a potential appears on the body of the electrical appliance due to damaged insulation, there is no current leakage, and when you touch it and grounded metal parts (heating radiator, water pipes), you can get a noticeable electric shock. In this case, the protective device will work, but it will be better if it comes from leakage into the ground.
For reliable operation of the protective device, grounding must be laid. When working according to this scheme, the RCD will break the circuit even beforetouching the metal case of equipment or household appliances.
Types of RCDs
RCDs are classified according to their functions:
- AC - response to a suddenly appearing or gradually increasing alternating leakage current.
- A - additionally triggers a constant pulsating differential current, which may appear unexpectedly or increase gradually.
- B - response to direct and alternating pulsating leakage currents.
- S - selective RCD with additional time delay for tripping.
- G - similar to S, but with less delay.
Which RCD to choose?
Ripple current in domestic conditions appears from washing machines, lighting dimmers, TVs, computers, power tools and other devices with switching power supplies. The absence of isolating transformers in devices with thyristor control significantly increased the likelihood of leakage of direct or alternating pulsating current. Therefore, if earlier it was enough to set the AC type, now type A or B is needed.
Where to install the RCD?
- Publicly accessible places in buildings where there is no increased risk of electric shock.
- In electrical circuits with a possible risk of electric shock (rooms with excessive humidity, outlet groups, household appliances, etc.).
- On the main input to protect against fire danger. Usually a selective RCD is installed here.
- In storey switchboards, in apartment panels, in individual houses.
- Bradial power supply systems: general selective RCD and separate ones for branch lines, with a choice of parameters that guarantee selective operation.
- At the near stages of protection, for example, 10 and 30 mA, 30 and 40 mA, etc., the current selectivity of the RCD is unlikely due to the high response speed. For the indicated values, it is provided if you select a selective RCD of 100mA so that there is still a time delay.
- Due to the aging of the insulation, there is not always a gradual increase in leakage currents.
- With an instant increase in leakage current due to insulation breakdown, any conventional RCD in series in the circuit can trip. This happens due to the rapid and significant exceeding of the settings at once on several stages of protection.
Need to use selective RCDs
Selective RCD performs its function of fire protection, if you use modifications with a time delay - S or G. They are subject to increased requirements for short circuit resistance, switching capacity, dynamic and thermal resistance, etc.
Usually, a selective fire protection RCD with a high leakage current is installed at the main input.
RCD must not be used in circuits that must not be suddenly turned off, as this can lead to emergency situations (fire or burglar alarm, danger to personnel, etc.).
In addition to RCDs, circuit breakers must have current selectivity. Should be the firstoperate closer to the area of overload or short circuit. In this case, the circuit breakers operate before the short-circuit current reaches the limit value. This is necessary to prevent overloading the series-connected sections, since the current passes through the contacts of their protective devices.
Types of selective RCDs
For a selective RCD, it is important to pause in order for the general type device located below the diagram to operate. In this case, the device with a time-delayed shutdown passes a leakage current through itself and does not work. The delay interval for models may vary. For products marked S, it is 0.15-0.5 s, for example, RCD 63a 100mA is selective, with the ability to adjust the delay. The choice will be the best if they are installed at the input of the apartment's power cable. Some foreign models have even higher time delays. They are designed to turn off the circuit in the event of a fire hazard. The longer the protection is turned off, the more likely it is to ignite the insulation.
When G is marked, the device fires within 0.06-0.08 s. The device is fast enough to respond to network problems. It should be installed below RCD type S selective. With two-stage protection, it can be installed on the main input, since the speed of the RCD connected below is still higher.
If there are several load groups in the network, a separate protective device is connected in front of each, and a selective one is connected to the inputfire protection RCD. Then, if one of the lines fails, only it will be de-energized, and the rest will remain connected. With a similar wiring diagram, it is easier to detect a malfunction. If a conventional RCD fails or does not respond to circuit problems, then a selective RCD (300 mA or 100 mA) will trip and shut down the entire network.
To ensure selectivity, the following instrument settings are required:
- set the tripping time of the selective RCD, if it provides for such a possibility;
- set the required trip parameters depending on the magnitude of the leakage current.
The tripping characteristics of selective RCDs must be at least 3 times larger than the rest. Only in this case the device will be guaranteed to work.
RCD parameters
Two RCD timing parameters are defined by Russian standards:
- breaking time - the interval from the appearance of the breaking leakage current ∆i to the moment the arc is extinguished;
- Break-off time limit for type S device - the time interval between the onset of ∆i and the opening of the contacts.
The last parameter determines the selectivity of the RCD. Its limit value is 0.5 s. In this case, it should be taken into account that for the protection of people, opening should occur within 10-30 ms, to prevent ignition of the insulation - up to 500 ms. RCD type S selective is widely used where it is necessary to exclude false alarms from the influence of interference or power surges.
According to the speed of disconnection of the RCD networkare separated as follows:
- general use - no delay;
- type G - 10-40ms;
- S type - 40-500ms.
Leakage currents always occur in electrical circuits. In sum, they should not exceed 1/3 of the nominal ∆i of the device. It is believed that for 1 A of the load there is 0.4 mA of consumer leakage current, and for 1 m of the length of the phase wire - 10 μA. The protective device is adjusted according to the total natural leakage current. If this is not done, frequent false positives may occur. In this case, it should be borne in mind that a device with ∆i=100 mA will no longer protect a person from electric shock.
When designing electrical networks, you can not specify the type of RCD until experts require it. But you need to justify your choice in advance. It is important that the rated current of the device is higher than the expected load current. In addition, the RCD is installed only in a common pair with a circuit breaker. You can install one differential machine instead of two devices. It will cost less, but you should choose the right parameters.
RCD protects in two-wire networks where there is no protective conductor. But it works only after touching a dangerous place.
Which fire RCD to choose?
Selective RCD 63A, 300mA is usually installed at the input as fire protection.
Many use conventional generic models with 30mA protection devices installed in the home. Here, the "partial" selectivity function is performed due to the large differenceoperating currents. This saves money on the price difference. In addition, a conventional RCD provides better safety due to faster response when catching leakage currents. The difference in the behavior of the devices is that the selective device will not trip first at a differential current equal to or greater than 300 mA. This situation is already extraordinary and there is no question of whether to go to the control panel, which may be located on a street pole. With such a large current, an ordinary RCD will probably work if an accident occurs on the line. Here and so it will be clear where to look for a malfunction.
Thus, a fire RCD can be installed both selective and conventional.
RCD Manufacturers
The Legrand Group is a world-renowned manufacturer of building electrical systems. The leading positions are ensured by the highest production culture and large investments in the creation of new electrical products. For Russia, the group supplies the entire range of electrical equipment, from sockets and switches to the most complex control systems.
Legrand selective RCD is of electronic and electromechanical type (indicated on the front panel). Depending on the version, it is installed on the side or below the circuit breakers. Adjustable time delay (0-1, 3 s) and sensitivity. In combination with machine guns, they are used as highly sensitive or basic protective devices.
RCD prices remain high, like other brands.
By ABB, RCDs are most fully represented by the F 200 series - from 16 A to 125 A. For a home network, RCD 63A, 100mA selective is sufficient. For leakage currents for household appliances, a 30 mA device is usually used. As a fire protection at the input of a private house, a selective RCD ABB (63A, 300mA) four-pole for a three-phase network is used, as one of the most reliable. It is not inferior in quality to Legrand brand products. For an apartment with a single-phase input, there will be a two-pole device. The photo below shows a selective RCD ABB 63A, 300mA.
The maximum current that the device can withstand is from 3 to 10 kA (indicated on the front panel). It is short-term, not working current. The RCD is able to pause until the machine disconnects the circuit.
The company is one of the leading, but the prices are very high. Consumers often prefer abb models because safety is the most important thing. ABB DDA200 AP-R type A and AC differential block is available. It has a trip delay of 10ms, although it is not an ABB selective RCD. Its trip characteristic curve is located between selective and conventional RCDs. The device has increased resistance to false alarms compared to general purpose devices.
The rejection rate for ABB selective RCD, as well as for other products, is only 2%, due to which there are practically no problems in operation. Electromechanical devices are much more reliable than electronic ones and have advantages in everything, forprice exception. RCDs with an electronic actuator are already beginning to appear, not inferior in reliability to a mechanical one.
On the market you can find products that are half the price, and the quality is not inferior to ABB. The company also produces the FH 200 series, which has a slightly lower price, but significantly loses in quality to the F 200 products. In particular, it does not have such reliable conductor fastening contacts that quickly begin to dangle, which affects the quality of work.
If you buy a selective ABB RCD, then only in specialized stores, and not in dubious places. A fake is dangerous because it is not able to protect a person properly. Modular equipment, which also includes RCDs, is paid great attention to by self-punchers because of the high cost.
The domestic group of companies IEK produces about 7,000 items of products that meet international standards and ensure reliable operation of power grids.
RCDs are subject to high demands. On the one hand, they must work reliably, protecting people from electric shock and wiring from the risk of ignition. But at the same time, devices installed at different stages of electrical circuits must act selectively, turning off individual sections. These conditions, as well as GOST 51326.1, correspond to the selective RCD IEK type VD1 63S.
The product group is represented by rated currents of 25-80A, and the residual currents are 100mA and 300mA. Products are cheaper than those of famous brands and are widelyused as introductory fire fighting apparatus. At the same time, protection selectivity is ensured by high cut-off currents and time delays for disconnecting circuits.
Selection of protective devices
When electricity is consumed in a simple way, a sinusoidal current flows through the circuit. The leak will be of a similar shape and AC type devices can be used here.
In modern household appliances, phase-cut control circuits are increasingly used. An AC type device will not respond to them, and here it is better to use an RCD of type A, which also responds to a sinusoidal current. The devices can be used together, for example, type AC is suitable for lighting with incandescent lamps, and type A is suitable for sockets to which devices with pulse regulation can be connected. But if you have to change the lighting to energy-saving lamps with brightness control by phase cut-off, you will also have to replace AC type device with A. Otherwise, it will not work.
In order to separate the operation by the levels of electrical circuits, it is necessary to use selective devices. Type S is installed on the main input, type G is installed on the second level, and then instantaneous devices.
RCD is selected one step higher in rated current than the circuit breaker connected in tandem with it, which can operate for a long time when the load is exceeded. If there is a 50 A machine at the input, a selective RCD 63A will suit it.
According to the requirements of the standards, the nominal voltage values are indicated on the front panels of the instruments, andalso continuous and breaking current ∆i. If there is a designation of a sinusoid, this is the AC type. The presence of two positive half-cycles under it means type A. Selective RCDs are denoted by the letters S and G. The rated short-circuit current is indicated in the box. The device must withstand its increase to the maximum until the machine turns off. Usually the current does not have time to reach the limit value. The RCD disconnects the circuit with a defect in advance, until the conductor has heated up and the insulation has not ignited.
Conclusion
In electrical household networks, current and time selectivity is used. To do this, protective devices are installed in series according to a tree diagram, where one switch is common. The basis of the principle of operation is to reduce the time of current flow through the body with direct or indirect contact with the elements of electrical installations under voltage. RCD selective is installed at the entrance and performs a fire-fighting function.
